One proposed structure for a secondary battery used in power output apparatuses is a design satisfying a relation of WHC/T≦50 (mm), where H, T, and W respectively denotes a width, a lamination height, and a depth of a flat electrode assembly, and C represents a clearance between a flat face of the flat electrode assembly and an inner wall of a rectangular container (see, for example, Patent Document 1). One typical example of such a secondary battery is a lithium ion battery satisfying the above relation. The size of the clearance ensuring the favorable results is selectable according to this relation.
Patent Document 1: Japanese Patent Laid-Open No. 2004-47332